Fans are well know in the prior art. Their designs, elements and materials vary depending upon the application. Typical applications of fans are ventilation and cooling for industrial or residential use. For instance, typical cooling and ventilation in industrial applications may be cooling towers, air-cooled condensers, cooling in petrochemical applications, heat exchangers, road tunnels, mining, subways, large buildings, etc.
Basically, a fan comprises at least one fluid reaction surface, usually called ‘blade’, and a rotor hub wherein the blade is connected. The blades and rotor hub may be manufactured in a single piece, usually for very small fans. Nevertheless, industrial applications usually require larger fans, whose blades and hub are separate pieces that must be connected together. The term ‘connection’ or ‘connecting’ is used herein in a broad sense meaning putting the pieces together, fixing, attaching, coupling, joining, fastening or the like. There are many systems for connecting a fan blade in the prior art, for instance BRPI0302441-5, BRPI0302858-5, BRPI8900333-0, BRPI0201725-3, BRMU8200206-1, BRMU8200229-1, BRMU8200250-9; WO97/41355; JP2000-314392; JP07-012096; JP63085202; JP11022696; JP63309404, U.S. Pat. No. 5,458,465 and GB843995.
The adopted connection system may determine important features of the fan related to the assembly, operation and maintenance, such as: adjustment of the pitch angle, the fan blade substitution or fan blade adjustment without interference in the other blades, the blade substitution or blade adjustment with the fan mounted inside the equipment where it will be operated, etc.
Blades for industrial applications are usually made of metal or of a composite material. In such cases, the blades can be massive or hollow, as well as they can possess characteristics of shells. The composite material of the blade can be, for instance, a fibre reinforced laminate, or a fibre reinforced plastic (FRP), or other filament and resin composite. Composite blades usually have the advantage to add to all the advantages of blades in composed material in relation to the blades in metallic material, such as the possibility of optimization of the blade geometry, light weight, high resistance and bigger damping of vibrations. In the case of blades made of composite materials, the connection system usually comprises a projected integral portion in the blade also in composite material or in a metallic material, said projected portion being connected to the rotor hub. In the case of metallic blades, threaded elements or welding are commonly used to make the connection.
There is a special kind of fan, more commonly known as ‘wind turbine’, which is used for the particular purpose of converting kinetic energy of the wind into mechanical energy. When this mechanical energy is used directly by the machinery (e.g. pumps), the wind turbine is usually called windmill; when the mechanical energy is converted into electricity, the wind turbine is called ‘wind generator’ or simply ‘wind energy turbine’. Due to the differences in the purposes, the rotor hub and the blades of medium or large wind energy turbine usually comprise significant differences from a rotor hub of a fan for a medium or large industrial application. There are many systems for connecting a wind turbine blade in a wind turbine rotor hub, for instance U.S. Pat. No. 4,260,332, U.S. Pat. No. 4,915,590, U.S. Pat. No. 6,371,730; WO01/79705; JP3015669; JP8093631; JP8270540 and JP11182408.